Optical Isolator, Shutter, Variable Optical Attenuator and Modulator Device And Method

ABSTRACT

An integrated optical device functioning as optical isolator, shutter, variable optical attenuator, and modulator is disclosed. The device employs a Pockels cell for dynamically rotating with nanosecond speed the polarization state of incident light for attenuation and modulation. The invention provides a compact, high performance and reliable device without moving parts for use in laser systems and particularly in fiber optic telecommunication system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/329,928 filed on Dec. 8, 2008 and titled An Optical Isolator,Shutter, Variable Optical Attenuator and Modulator Device, whichapplication is incorporated by reference into this application.

FIELD OF THE INVENTION

This invention relates to a free-space, polarization-dependent opticalisolator, shutter, variable optical attenuator, and modulator device,and more particularly to a device which uses an optical Pockels cell,and Faraday rotator to control the optical polarization state.

DESCRIPTION OF THE RELATED ART

An optical isolator, shutter, variable attenuator, and modulator are allwidely used in laser systems and particularly in modern fiber opticaltelecommunication networks.

An optical isolator eliminates unwanted or reflected optical signalsfrom interfering with a desired optical function. In fiber opticcommunications systems, some light may be reflected back from the fibernetwork. This reflected light affects the operation of the laser diodeby interfering with and altering the frequency of the laser outputoscillations. For this reason, an optical isolator is typically providedbetween the laser diode and the optical fiber to minimize the reflectionfrom the fiber network.

FIG. 1A shows a conventional optical isolator configuration as disclosedin the prior art. The isolator includes two polarizers (14-1 and 14-2)and a Faraday rotator 16 disposed between the polarizers 14-1 and 14-2.FIG. 1B shows the polarization plane orientation of two polarizers withpolarization plane 11 of first polarizer 14-1 aligned with the x-axisand polarization plane 15 of second polarizer 14-2 aligned at 45 degreeangle from x-axis. After passing through the first polarizer 14-1, theFaraday rotator 16 rotates the polarization of the input beam of light10-1 by 45 degrees, so that the beam of light 10-2 can then pass withoutchange through the second polarizer 14-2.

The rotation of the polarization plane provided by the Faraday rotator16 for a light ray traveling in one direction allows light to passthrough both polarizers 14-1 and 14-2, whereas for a light ray travelingin the direction opposite to beam 10-1, the plane of polarization isrotated so that the passage of the back reflected light of light 10-3through the Faraday rotator 16 is blocked by the first polarizer 14-1.

An optical attenuator is a very important element of an optical circuitfor controlling an optical signal transmission. In fiber opticcommunication systems, variable optical attenuators are broadly employedto regulate the optical power levels to prevent damage to the opticalreceivers caused by irregular optical power variations. As the opticalpower fluctuates, a variable optical attenuator is employed, incombination with an output power detector and a feedback control loop,to adjust the attenuation and to maintain at a relatively constant levelthe optical power transmitted to a photo-receiver. Optical signalattenuation can be accomplished in a variety of ways by diverting all ora portion of an optical signal from an original pathway.

Variable optical attenuators (VOA) have been developed with a variety oftechnologies. Currently, there are several types of commerciallyavailable variable optical attenuators in the market, such asopto-mechanical VOA devices using stepping motor or magneto-opticalcrystal, devices using liquid crystal (LC) technology, and devices usingmicro-electro-mechanical systems (MEMS) technology.

Optical shutters or switches implemented with Pockels cells, which arebased on an electro-optical crystal's birefringence, are usually foundin non-telecommunications applications, mainly because of the cell'svery high voltage requirement. The Pockels electro-optic effect producesbirefringence in an optical medium by a constant or varying electricfield. The electric field can be applied to the crystal medium eitherlongitudinally or transversely to the light beam. Longitudinal Pockelscells need transparent or ring electrodes. Transverse voltagerequirements can be reduced by lengthening the crystal. A Pockels cell26 combined with two polarizers 24-1, 24-2 can be used for a variety ofapplications. FIG. 2A shows a simple configuration of a Pockelscell-based device having multiple functions such as an optical variableattenuator and modulator as disclosed in the prior art.

FIG. 2B shows the polarization plane orientation for a normally closedshutter (the shutter is closed without applied voltage) of twopolarizers with the polarization plane 21 of the first polarizer 24-1,in FIG. 2A, aligned with x-axis, and the polarization plane 25 of secondpolarizer 24-2 aligned with y-axis. When a variable electric fieldgenerated by Pockels cell driver 29 has a value between zero and thehalf-wave voltage, i.e., the voltage under which the plane ofpolarization of incident light is rotated 90 degrees by Pockels cell,the input light beam 20-1 can be attenuated from completely off tocompletely transparent when light beam 20-2 exits polarizer 24-2 aslight beam 20-3.

Such a configuration is not often used in fiber opticaltelecommunication system mainly because of the extremely high voltage(half wave voltage is about a few thousand volts or even higher)requirements, even though the response time is ultrafast, on the ordernanoseconds. However, with the development of the new materials, thevoltage required to create birefringence has been substantially reduced,and therefore the present invention is a viable approach for fiberoptical telecom networks, especially in transmitters with capability ofdirectly modulating the signal emitted from at laser at very highfrequencies and low voltage.

BRIEF SUMMARY OF THE INVENTION

One embodiment is an integrated, multifunctional optical device. Thedevice includes a first and second polarizer, a Pockels cell, a Faradayrotator, at least one electric driver for the attenuator and shutter andan electric driver for the modulator. Each of the polarizers has aparticular polarization plane. The polarization plane of said secondpolarizer is oriented at a 45 degree angle to the plane of said firstpolarizer. The first polarizer receives the input light beam and thesecond polarizer provides output light beam. The Pockels cell isconfigured to rotate an input beam based on an external voltage. TheFaraday rotator disposed between the first polarizer and the Pockelscell. The Pockels cell is disposed between said Faraday rotator and saidsecond polarizer to receive the input beam from the Faraday rotator. ThePockels cell has two birefringent axes aligned at 45 degreessymmetrically to the polarization plane of the said second polarizer.One electric driver is configured to drive the attenuator and theshutter. Another electric driver is configured to drive the modulator.

Another embodiment is a method for providing a modifiable intensityoutput beam from an input light beam. The method includes the steps ofpolarizing the input light beam along a first plane of polarization,rotating the polarized input beam by a Faraday rotator to generate afirst rotated beam, where the first rotated beam is rotated by an angleof about 45 degrees relative to the first plane of polarization,rotating by a Pockels cell the first rotated beam in response to anexternal input voltage to generate a second rotated beam, where thesecond rotated beam rotated by an angle between 0 and 90 degreesrelative to the first rotated beam depending on the external inputvoltage, polarizing the second rotated beam to generate the modifiableintensity output beam having a maximum intensity when the second rotatedbeam is rotated by 0 degrees, zero or close to zero intensity when thesecond rotated beam is rotated by 90 degrees, and variable intensitywhen the second rotated beam is rotated in a range between 0 degrees and90 degrees.

It is an object of the present invention to provide an integrated,compact and multi-functional optical device operating as opticalisolator, shutter, variable attenuator, and optical modulator withoutmoving parts for applications in variety of laser systems and, inparticular, in fiber optic telecommunication networks.

It is also an object of the present invention to provide ultrafast(nanosecond level) variable attenuation and shuttering of opticalsignals.

With such a simple and integrated design leading to bettermanufacturability, it is further an object to make such a device massproducible at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1A shows an optical isolator design utilized in the prior art;

FIG. 1B illustrates the polarization plane orientation of two polarizersin an isolator design;

FIG. 2A shows an attenuator design utilized in the prior art;

FIG. 2B illustrates the polarization plane orientation of two polarizersin the attenuator design;

FIG. 3A shows an embodiment of present invention for an integrateddevice with multiple functions;

FIG. 3B illustrates the polarization plane orientation of two polarizersin an embodiment of the present invention; and

FIG. 3C illustrates the orientation of two axes of the birefringentPockels cell with respect to the polarization plane orientation of twopolarizers.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the configuration of an integrated deviceperforming the functions of isolator, attenuator, shutter, and modulatoris schematically shown in FIG. 3A. For easy understanding of thedescription, the axes of coordinates are set as follows. Let az-direction (toward the right in the drawing) indicate the direction inwhich the optical components are aligned and let the x-direction(vertical direction) and y-direction (horizontal direction) indicate thetwo directions orthogonal to the z-direction. The polarization plane 31of the first polarizer 34-1 is aligned with the x-axis, and thepolarization plane 35 of second polarizer 34-2 is aligned at 45 degreesto the x-axis as shown in FIG. 3B. In this system, the collimated inputlight beam 30-1 is substantially linearly polarized, substantiallycoherent, monochromatic and collimated. The input light beam 30-1 isalso oriented with the polarization axis of the first polarizer 34-1 topass through the first polarizer 34-1, which performs the Jones matrixoperation

$\begin{bmatrix}0 & 0 \\0 & 1\end{bmatrix}.$

The polarization orientation of light beam 30-2 is rotated 45 degrees(in x and y plane) after passing through the Faraday rotator 36, i.e.,

${1/{2\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}}},$

which is designed to work for the prescribed wavelength of the inputlight since the rotation angle by Faraday rotator is wavelengthdependent. Typically, the Faraday rotator is configured for a singlewavelength or within a certain wavelength range to meet the wavelengthrequirement for the application.

The Pockels cell 37 includes a transparent isotropic or non-birefringentmedium. The Pockels cell medium is selected to meet specific wavelengthrequirement according to the application. Without an applied electricfield, the cell 37 allows the light 30-3 to pass through the secondpolarizer 34-2, which is oriented at 45 degrees from the polarizationplane 31 of first polarizer 34-1. Under these conditions, for the inputlight propagating in the z-direction, the system is transparent to theinput light beam 30-1 but blocking to the reflected light of light beam30-4. The configuration depicted in FIG. 3A operates in a fashionsimilar to the isolator depicted in FIG. 1A which provides substantialattenuation in a backward direction.

When the electric field generated by driver 32 and/or 33 is applied, thePockels cell 37 becomes a voltage-controlled wave plate, i.e.,

${^{\; {\theta {(V)}}}\begin{bmatrix}1 & 0 \\0 & {\pm i}\end{bmatrix}},$

with birefringent axes 39-1 and 39-2 aligned with x-axis and y-axis asshown in FIG. 3C. In order for the system to work as a shutter, driver32 applies a voltage V sufficient to make Pockels cell 37 become a halfwave plate, i.e.,

${^{\; {\theta {(V)}}}\begin{bmatrix}1 & 0 \\0 & {\pm i}\end{bmatrix}},$

with

${{\theta (V)} = \frac{\pi}{2}},$

which rotates the polarization direction of the input light 30-2 exitingfrom the Faraday rotator 36 by 90 degrees in x-y plane. Such a voltageis usually called half-wave voltage. The light beam 30-3, with itspolarization plane 90 degrees to that of the polarizer 34-2, is thencompletely blocked by the polarizer 34-2. Due to the ultrafast responsetime of a typical Pockels cell, such a shutter can be made withnanosecond switching time as disclosed in the art.

With the applied voltage less than the half-wave voltage, the systemacts as an attenuator. By varying the applied voltage, the intensity ofthe incident light beam 30-1 reaching the output 30-4 via the secondpolarizer 34-2 varies from being fully transmitted to being completelyblocked. In practice, some insertion losses are incurred even withoutthe applied electric field due to the material absorption, scattering,reflection, and misalignment of the polarization axes, etc. It should benoted that even though the back reflection isolation of light beam 30-4of the preferred system configuration is degraded when the system isused as an attenuator, the back reflection is also substantially reducedfor the attenuated input light beam 30-1. Therefore, the total backreflection isolation for the system is not significantly sacrificed. Itis clear that the system can modulate the input light 30-1 when Pockelscell 37 is driven by modulator driver 33. Due to the high half wavevoltage needed, a high modulation frequency is difficult to achieve foron-off states, but small amplitude modulation of light 30-1 ispractical.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, the device can be configured as a normally-closedshutter (i.e., the device will completely block the input light beamwithout applying voltage to Pockels cell 37) by rotating thepolarization orientation plane 35 of the second polarizer 34-2 90degrees from its normally-open position depicted on FIG. 3B. In anotherversion, all facets of the optical components including the polarizers,the Faraday rotator, and the Pockels cell can be coated with multiplelayers of anti-reflection dielectric thin films to eliminate thereflections and reduce the light insertion loss. Additionally, the firstpolarizer, the Faraday rotator, the Pockels cell and the secondpolarizer can be bonded with adhesives that are transparent to theselected optical wavelength, or bonded by adhesives without covering orinterfering with areas through which the light beam passes. Therefore,the spirit and scope of the appended claims should not be limited to thedescription of the preferred versions contained herein.

What is claimed is:
 1. A method for providing a modifiable intensityoutput beam from an input light beam, the method comprising: polarizingthe input light beam along a first plane of polarization; rotating thepolarized input beam by a Faraday rotator to generate a first rotatedbeam, wherein the first rotated beam is rotated by an angle of about 45degrees relative to the first plane of polarization; rotating by aPockels cell the first rotated beam in response to an external inputvoltage to generate a second rotated beam, where the second rotated beamrotated by an angle between 0 and 90 degrees relative to the firstrotated beam depending on the external input voltage; and polarizing thesecond rotated beam to generate the modifiable intensity output beamhaving a maximum intensity when the second rotated beam is rotated by 0degrees, zero or close to zero intensity when the second rotated beam isrotated by 90 degrees, and variable intensity when the second rotatedbeam is rotated in a range between 0 degrees and 90 degrees.
 2. Themethod of claim 1, wherein the input light beam is substantiallylinearly polarized, substantially coherent, monochromatic andcollimated, and oriented with the first plane of polarization.
 3. Themethod of claim 1, wherein the Faraday rotator is configured for asingle wavelength or within certain wavelength range to rotate thepolarized input light beam by an angle of 45 degrees of the firstpolarization plane.
 4. The method of claim 1, wherein the Faradayrotator is selected to meet a wavelength requirement for a particularapplication.
 5. The method of claim 1, wherein the Pockels cell includesan electric field sensitive medium; wherein the external input voltagesupplies energy for an electric field applied to the electric fieldsensitive medium of the Pockels cell; and wherein the electric fieldapplied to the Pockels cell electric field sensitive medium is eitherlongitudinal or transverse to the light beam input to the Pockels cell.6. The method of claim 5, wherein the Pockels cell electric fieldsensitive medium is selected to meet a specific wavelength requirementfor a particular application.
 7. The method of claim 1, wherein theFaraday rotator and the Pockels cell have a number of facets; andwherein all facets the Faraday rotator and the Pockels cell are coatedwith multiple layers of anti-reflection dielectric thin films toeliminate reflections and reduce light insertion loss.
 8. The method ofclaim 1, wherein light beams pass respectively through the Faradayrotator and the Pockels cell; and wherein the Faraday rotator and thePockels cell are bonded with adhesives that are transparent to theselected optical wavelength, or bonded by adhesives without interferingwith areas through which the light beam passes.